1. Field of the Invention
The present invention generally relates to an electrostatic discharge (ESD) protection circuit, in particular, to a Charged-Device Model (CDM) ESD protection circuit.
2. Description of Related Art
An ESD event refers to a phenomenon of electrical discharge of a current for a short duration during which a large amount of current is provided to a semiconductor integrated circuit (IC). ICs are generally susceptible to ESD events, which may damage or destroy the integrated circuit. Thus, ESD protection of ICs is a critical factor in obtaining high yield and stable IC characteristics. The susceptibility of a device to ESD can be determined by testing each one of three models which include Human Body Model (HBM), Machines Model (MM) and Charged-Device Model (CDM).
With regard to a CDM ESD event, electrostatic charge could be stored within the body of an IC product due to induction or tribocharging and most of the charge is accumulated in a substrate, including a base, a body or a well of the devices disposed on the IC, and is uniformly distributed in the substrate. Once a certain pin of the IC is suddenly grounded, the electrostatic charge originally stored within the IC will discharge through the grounded pin. This is called the CDM ESD event. The CDM ESD event delivers a large amount of current in a very short time, and in general the entire ESD event can take place in less than 2 nanoseconds (ns). Current levels can reach several tens of amperes during discharge, which are remarkably greater than those of the HBM and MM models.
Additionally, there are many situations where the pins of an IC may become grounded, for example, the pin may touch grounded metallic surface or the pin may be touched by grounded metallic tools. Different ICs have different die sizes, so their equivalent parasitic capacitances are totally different from one another. Thus, different ICs have different peak current and different CDM ESD levels. When a device under test (DUT) with the equivalent capacitance of 4 pF is under a 1-kV CDM ESD test, the CDM ESD current can rise to more than 15 A within several nanoseconds. Compared with HBM and MM ESD events, the discharging current in a CDM ESD event is not only larger, but faster. Since the duration of CDM ESD events is much shorter than HBM and MM ESD events, the internal circuit may be damaged during CDM ESD events before the ESD protection circuit is turned on.